1. Field of the Invention
This invention relates to processes and apparatus for the fluidized contacting of catalyst with hydrocarbons. More specifically, this invention relates to the processes and apparatus for stripping entrained or adsorbed hydrocarbons from catalyst particles.
2. Description of the Prior Art
A variety of processes contact finely divided particulate material with a hydrocarbon containing feed under conditions wherein a fluid maintains the particles in a fluidized condition to effect transport of the solid particles to different stages of the process. Catalyst cracking is a prime example of such a process that contacts hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The hydrocarbon feed fluidizes the catalyst and typically transports it in a riser as the catalyst promotes the cracking reaction. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone burns coke from the catalyst by contact with an oxygen-containing stream that again serves as a fluidization medium. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known to those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbons retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that enter the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the combustion load in the regeneration zone. Higher combustion loads lead to higher temperatures which at some point may damage the catalyst or exceed the metallurgical design limits of the regeneration apparatus.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, counter-current to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and adsorbed on the catalyst. Contact of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and counter-currently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. No. 2,440,620, U.S. Pat. No. 2,612,438, U.S. Pat. No. 3,894,932, U.S. Pat. No. 4,414,100 and U.S. Pat. No. 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of outer baffles in the form of frusto-conical sections that direct the catalyst inwardly onto a series of inner baffles. The inner baffles are centrally located conical or frusto-conical sections that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffles and continues rising upwardly from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated in FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
It is an objective of any new stripping design to minimize the addition of stripping medium while maintaining the benefits of good catalyst stripping throughout the FCC process unit. In order to achieve good stripping of the catalyst with the resultant increased product yield and enhanced regenerator operation, relatively large amounts of stripping medium have been required. For the most common stripping medium, steam, the average requirement throughout the industry is about 2 lbs. of steam per 1000 lbs. (2.0 kg of steam per 1000 kg) of catalyst for catalyst stripping. In the case of steam, the costs include capital expenses and utility expenses associated with supplying the steam and removing the resulting water via downstream separation facilities. Where there is not adequate supply or treatment capacity, the costs associated with raising the addition of stripping medium can be significant. In such cases achieving better stripping without an increase in the required steam will yield substantial economic benefits to the FCC process.
However, better stripping brings more important economic benefits to the FCC process by reducing coke production. Reducing coke production permits a lowering of the regenerator temperature so that the reaction may operate at a higher catalyst-to-oil (C/O) ratio. The higher C/O increases conversion and increases the production of valuable products. A stripping operation that reduces the production of coke by 0.05 wt-% can lower regenerator temperature by 15xc2x0 to 20xc2x0 F. (xe2x88x929xc2x0 to xe2x88x927xc2x0 C.) and permit a C/O ratio increase in the range of 6%. The corresponding improvement in conversion yields 0.6 to 0.7 vol-% more gasoline as well also increasing the yield of desired light products. Therefore, it is a further objective of this invention to decrease coke production by more efficient catalyst stripping.
Moreover, it is not possible to simply increase stripping efficiency or capacity by accepting the economic penalties associated with the use of increasing amounts of steam. At some point, the typical stripper that operates with baffles becomes limited by the amount of catalyst flux moving through the stripper. A practical limit on catalyst flux for operating such strippers is approximately 90,000 lbs/hr/ft2 (439,380 kg/hr/m2) based on total stripper area. Attempts have been made to increase the capacity and effectiveness of stripping in a baffle-style stripping unit by modifying the configuration and area of the baffles. U.S. Pat. No. 5,531,884 shows a modification to a baffle-style stripper that incorporates one or more rings of large vertical conduits to provide an additional catalyst and gas circulation path across the baffles. It is also known to concentrate opening in a very centralized portion of the stripping baffles.
It has now been found that providing a baffle-style stripper with a complete or nearly complete coverage of stripping openings over the sloped surface of the baffle will provide improved stripping efficiency and catalyst flux through the stripper. It was also unexpectedly found that the stripper efficiency increases with higher catalyst flux when using the modified baffles of this invention. The complete distribution of relatively small openings over the entire surface of a sloped baffle has been found to interrupt relatively dense streamers of catalyst that were previously unrecognized to exist and which short-circuited the contact of the stripping fluid with the catalyst. The presence of these streamers would permit the catalyst to take the shortest possible path across the sloped surface of the baffle from the point where the catalyst first contacts the stripper baffle to the point where catalyst exits the bottom of the baffle. This flow pattern leaves large areas of the stripper inactive and containing dense slumped catalyst. Spreading out the stripping gas across the sloped area of the baffle to a much greater extent than has been practiced in the past has now been found to promote active contacting of the catalyst with the stripping fluid over the entire volume of the stripper between the baffles. As an added benefit more complete coverage by the stripper opening also prevents choking of stripper flow by the restriction of stripping gas flow to narrow open areas between the sloped baffles. By this discovery, previous limits for typical baffle-type stripper throughput may be increased by as much as 50%. The process of this invention has particular benefits at flux rates of at least 90,000 lbs/hr/ft2 (439,380 kg/hr/m2) of stripper area and is particularly useful at flux rates over 120,000 lbs/hr/ft2 (585,840 kg/hr/m2) of stripper area and even over 140,000 lbs/hr/ft2 (683,480 kg/hr/m2) of stripper area.
Accordingly, it is an object of this invention to increase the maximum capacity at which a baffle-style stripper may operate.
It is another object of this invention to increase the efficiency of stripping in a baffle style stripper.
It is a further object of this invention to obtain a method and apparatus that provides a more complete utilization of stripping medium.
While not wishing to be bound to any theory, it is believed that relatively small openings on the order of 1.5 inches (3.8 cm) or less distributed to cover essentially the entire sloped baffle surface effect better stripping than larger, less dispersed openings that leave greater areas of the sloped baffle surface without openings for the fluidization medium. Smaller diameters minimize the length of jet penetration of the fluidization medium into the catalyst on the surface of the baffle. Since it is believed that mass transfer takes place in the zone around the jet and the bubble of gas formed by the jet, the most effective stripping should occur when the small jets create relatively small gas bubbles and provide greater interactive surface between the gas and the catalyst that it contacts. Smaller openings and resulting smaller bubble creation complements the concept of more opening dispersal to avoid any areas over the baffle surface that may be bypassed by the streamers of catalyst.
Larger hole sizes tend to create large jets of gas from the distribution holes. Long jet penetration into the catalyst above the baffle has the disadvantage of creating relatively large bubbles as the jet dissipates while also bypassing catalyst due to the high jet velocity and delaying the dissipation of gas bubbles that increases the contacting of the gas with the catalyst. Preferred holes for this arrangement have diameters ranging from 0.38 to 0.75 inch (1 to 1.9 cm), with smaller diameters being preferred for the purposes of gas and catalyst contacting. However, decreasing hole diameter has the disadvantage of increasing the possibility of hole plugging by catalyst, refractory or scale blockage in the hole. As a result, holes in a range of from 0.5 to 0.75 inch (1.3 to 1.9 cm) are particularly preferred as a compromise between minimizing jet length while avoiding plugging of the holes.
This invention uses a much greater dispersal of holes over the entire sloped surface of the baffle than has been employed in the past. The holes emit stripping gas that accumulates in the pocket formed by the underside of the sloped baffle. Distribution of the holes over the entire sloped surface means the elimination of the previous large areas that were left without fluidizing gas perforations. At a minimum, the distribution of the openings over the sloped surface will provide at least one opening for each square foot (0.09 square meter) of the sloped surface of each baffle. Preferably, this invention will eliminate any large unperforated areas of the sloped surface by minimizing areas that do not contain any perforations. One measure of eliminated unperforated areas will provide a baffle perforation pattern wherein subdivision of any portion of the sloped surface into a circular area of at least 1 ft2 (0.09 m2) will surround at least a portion of one or more openings in that area. Another measure of substantially complete distribution of perforations on the sloped surface has perforations within at least 8 inches (20.3 cm) of an adjacent perforation, preferably within 6 inches (15.2 cm) of an adjacent perforation and more preferably within 4 inches (10.2 cm) of an adjacent perforation. Furthermore, it is preferable to eliminate any large areas of unperforated sloped baffle surface at the top or the bottom of the baffles. One measure of eliminating these areas that were created by past hole distributions places a perforation within at least 6 inches (15.2 cm) of either the top or bottom baffle edge.
In addition, the distribution of holes across the baffle need not be uniform. The sloped surface of the baffle creates a higher differential pressure across the sloped surface of the baffle as the elevation of the holes along the sloped surface increases. Accordingly, a uniform distribution of the uniformly sized holes across the baffle will provide a greater volume of gas delivery across the higher elevations of individual baffles due to the greater jet length associated with the greater pressure differential. Therefore, as long as the baffle is sloped, a uniform volumetric delivery of gas over the baffle surface requires an increase in distribution hole area toward the lower part of the baffle. It is preferred that in high catalyst flux applications, in the range of 90,000 lbs/hr/ft2 (439,380 kg/hr/m2) or greater, the hole distribution pattern provides an even volumetric gas delivery across the sloped surface of the baffle. Such delivery can be provided by biasing the percentage of open hole area toward the lower part of the baffle. The open hole area may be increased by using a greater percentage of holes, larger holes, or both, in the lower baffle portion.
Even for applications where the catalyst flux through the stripping zone is relatively lower, the hole distribution arrangement of this invention still provides substantial benefits. However, for lower flux applications which typically refer to a flux below 90,000 lbs/hr/ft2 (439,380 kg/hr/m2) of stripper area, the catalyst flow across the stripping baffles tends to be greatest towards the lower portion of the baffles. Accordingly, a biasing of the gas flow towards the lower portion of the baffle can particularly benefit low catalyst flux applications. In such cases, it may be beneficial to increase the volume of open hole area towards the lower portion of the baffle beyond that which would provide a uniform volumetric gas delivery across the baffle such that a greater volumetric delivery of gas occurs over the lower portion of the sloped baffle.
We also found that setting stripping baffles at angles of 30xc2x0 and 60xc2x0 operated with comparable efficiency and sometimes better efficiency than baffles operated at angles of 45xc2x0, all incorporating the improved hole distribution of the present invention. Hence, baffle angles of 30xc2x0 may be preferable to baffle angles of 45xc2x0 or 60xc2x0 because more baffles can be assembled in a given height of stripping column and because there is lower pressure differential across the baffle oriented at shallower angles. Moreover, at a 30xc2x0 baffle angle, high efficiency was observed for flux rates as high as 140,000 lbs/hr/ft2 (683,480 kg/hr/m2) of catalyst.
In a broad embodiment, this invention is a process for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The process contacts particles with a hydrocarbon stream and disengages the hydrocarbons from the particles after contact with the hydrocarbon stream to produce a stream of contacted particles containing entrained or adsorbed hydrocarbons. The contacted particles pass downwardly through a plurality of sloped stripping baffles. A plurality of openings distributed over the entire sloped surface of each stripping baffle discharge stripping fluid upwardly through the baffle to strip hydrocarbons from the particulate material that passes over the top of the baffle. The arrangement of baffle openings provides at least one opening for each square foot (0.09 square meter) of the baffle sloped surface. The process recovers stripping fluid, stripped hydrocarbons, and stripped particles from the stripping baffles.
In a more limited embodiment, this invention is a process for the stripping of entrained and/or adsorbed hydrocarbons from particulate material where the entrained or adsorbed hydrocarbons are from the fluidized catalytic cracking (FCC) of an FCC feed with a particulate material comprising an FCC catalyst. The process contacts an FCC feed with FCC catalyst to provide a mixture of catalyst and feed and to convert the FCC feed while depositing coke on the FCC catalyst. Disengagement of the converted FCC feed from the FCC catalyst produces a stream of disengaged catalyst particles containing entrained or adsorbed hydrocarbon. The disengaged catalyst particle stream passes into a stripping zone and downwardly through a plurality of vertically sloped stripping baffles located in the stripping zone. A plurality of openings distributed over the entire sloped surface of each stripping baffle discharge stripping fluid upwardly across the baffle to strip hydrocarbons from the FCC catalyst that passes over the top of the baffle. The arrangement of baffle openings provides at least one opening for each square foot (0.09 square meter) of the baffle sloped surface. The process recovers stripping fluid and stripped hydrocarbons that pass upwardly from the stripping baffles and stripped FCC catalyst that passes downwardly from the stripping baffles. Stripped FCC catalyst passes to a regeneration zone to remove coke from the FCC catalyst. FCC catalyst from the regeneration zone is returned for contact with the FCC feed.
In an apparatus embodiment, this invention comprises a stripper arrangement for the stripping of entrained and/or adsorbed hydrocarbons from particulate material. The apparatus comprises a stripping vessel defining at least one port for receiving particles that contain entrained or adsorbed hydrocarbons from the contact of the particles with a hydrocarbon stream and for withdrawing stripping fluid and stripped hydrocarbons from the stripping vessel. A plurality of sloped stripping baffles are spaced apart vertically over at least a portion of the stripping vessel height with each baffle having a sloped surface and each sloped surface having a transverse projection equal to at least one-third of the minimum transverse cross-section of the stripping vessel at that baffle location. A plurality of openings distributed over the entire sloped surface of each stripping baffle provide at least one opening for each square foot (0.09 square meter) of the sloped surface. The openings are preferably provided within 9 inches (23 cm) of the edge of any baffle and more preferably within 6 inches (15 cm) of the edge of any baffle. The stripper includes at least one fluid inlet that passes a stripping fluid to the underside of one or more stripping baffle for stripping hydrocarbons from the particulate material. The stripping vessel also includes at least one particle outlet for recovering stripped particles from the stripping baffles. The port at the top of the stripping vessel will ordinarily serve as an inlet receiving particles and an outlet for withdrawing stripping gas and stripping fluid. However, separate inlets and outlets may be employed, in particular a separate inlet may be provided for passing the hydrocarbon containing catalyst into the stripper vessel.
Additional objects, embodiments, and details of this invention are given in the following detailed description of the invention.